Dynamical attribution of North Atlantic interdecadal predictability to oceanic and atmospheric turbulence under realistic and optimal stochastic forcing

Abstract

Unpredictable variations in the ocean originate from both external atmospheric forcing and chaotic processes internal to the ocean itself, and are a crucial sink of predictability on interdecadal timescales. In a global ocean model, we present i.) an optimisation framework to compute the most efficient noise patterns to generate uncertainty and ii.) a uniquely inexpensive, dynamical method for attributing sources of ocean uncertainty to internal (mesoscale eddy turbulence) and external (atmospheric) origins, sidestepping the more typical ensemble approach. These two methods are then applied to a range of metrics (heat content, volume transport, and heat transport) and time averages (monthly, yearly, and decadal) in the subtropical and subpolar North Atlantic.We demonstrate that optimal noise patterns target features of the underlying circulation such as the North Atlantic Current and deep water formation regions. We then show that noise forcing in the actual climate system stimulates these patterns with various degrees of efficiency, ultimately leading to the growth of error. We reaffirm the established notion that higher frequency variations are primarily wind driven, while surface buoyancy forcing is the ultimately dominant source of uncertainty at lower frequencies. For year-averaged quantities in the subtropics, it is mesoscale eddies which contribute the most to ocean error, accounting for up to 60% after 60 years of growth in the case of volume transport at 25 • N. The impact of eddies is greatly reduced in the subpolar region, which we suggest may be explained by overall lower sensitivity to small-scale noise there. 2Significance statement. Climate does not change steadily; it naturally fluctuates around a general trend. The prediction of climate several decades to a century ahead depends mostly on the ability to anticipate future human activity, but for the coming years to a few decades ahead (when the future pathway of human activity is not yet fully apparent) natural fluctuations also have an important role. These fluctuations, however, cannot be perfectly predicted for long. The ability to predict them is limited, for example, by the build-up of unwelcome "noise" from erratic processes such as the weather. In this study, we look at the different sources of this noise, how important they are, and how they impact prediction accuracy of climatically important ocean quantities decades in the future. To achieve this, we use a unique computer simulation of the ocean, which works backwards and describes how to most effectively create change. This uncovers the mechanisms by which noise is most effectively amplified by the ocean, and also shows how this compares with the behavior of noise in the real ocean-atmosphere system. We demonstrate that in the climatically important region of the North Atlantic, unpredictable ocean circulation changes in the more southerly tropical region are mostly due to oceanic mesoscale eddies (the oceanic equivalent of atmospheric storms).Further north, however, it is the atmospher...